1. Field of the Invention
The present invention relates in general to the automatic control of tracking, and more particularly to an automatic tracking control apparatus and method in which a gain of a tracking error signal is automatically adjusted so that tracks of a magnetic tape can accurately be traced by video heads.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional automatic tracking control apparatus. As shown in this drawing, the automatic tracking control apparatus comprises a switch SW1 for selecting one of recording and playback modes, a low pass filter (LPF) 1 for detecting a pilot signal from high frequency signals read from a magnetic tape by video heads HD in the playback mode, an amplifier 2 for amplifying the detected pilot signal from the low pass filter 1, a reference pilot generator 3 for dividing a frequency signal from an oscillator OS by N to generate reference pilot signals, where N is determined according to select signals SEL1 and SEL2, and a mixer 4 for generating tracking control signals of 16 KHz and 46 KHz frequency components in accordance with differences between the respective reference pilot signals from the reference pilot generator 3 and the amplified pilot signal from the amplifier 2.
A first frequency detector 5 is provided in the automatic tracking control apparatus to detect the tracking control signal of the 16 KHz frequency component from output signals from the mixer 4 and wave-shape the detected tracking control signal. A first level detector 8 is adapted to detect a level of an output signal from the first frequency detector 5.
A second frequency detector 7 is also provided in the automatic tracking control apparatus to detect the tracking control signal of the 46 KHz frequency component from the output signals from the mixer 4 and wave-shape the detected tracking control signal. A second level detector 8 is adapted to detect a level of an output signal from the second frequency detector 7.
A comparator 9 is also provided in the automatic tracking control apparatus to subtract output levels from the first and second level detectors 6 and 8 from each other and output a tracking error signal in accordance with the subtracted result.
A microprocessor 10 is also provided in the automatic tracking control apparatus to generate a capstan motor control signal in response to the tracking error signal from the comparator 9. Also, the microprocessor 10 generates the select signals SEL1 and SEL2 for the generation of the reference pilot signals resulting in the generation of the tracking control signals of the 16 KHz and 46 KHz frequency components and applies the generated select signals SEL1 and SEL2 to the reference pilot generator 3.
The operation of the conventional automatic tracking control apparatus with the above-mentioned construction will hereinafter be described.
In the playback mode, selected under the control of the switch SW1, the high frequency signals recorded on the magnetic tape are read by the video heads HD and the pilot signal is detected from the read high frequency signals by the low pass filter 1. The pilot signal is used to control a phase of a capstan motor M. The pilot signal from the low pass filter 1 is applied to the amplifier 2 for amplification thereof.
On the other hand, the select signals SEL1 and SEL2 from the microprocessor to are applied to the reference pilot generator 3 for the generation of the reference pilot signals resulting in the generation of the tracking control signals of the 16 KHz and 46 KHz frequency components. The reference pilot generator 3 divides the frequency signal from the oscillator OS by N determined according to the select signals SEL1 and SEL2 from the microprocessor 10, so as to generate the reference pilot signals. The reference pilot signals from the reference pilot generator 3 and the amplified pilot signal from the amplifier 2 are applied to the mixer 4, which generates the tracking control signals of the 16 KHz and 46 KHz frequency components in accordance with the differences between the respective reference pilot signals from the reference pilot generator 3 and the amplified pilot signal from the amplifier 2.
Noticeably, the tracking control signals placed on the 16 KHz and 46 KHz frequencies are mis-tracking information in the case where the video heads HD trace tracks of the magnetic tape leaning to the left and right of the corresponding tracks.
The output signals from the mixer 4 are applied to the first and second frequency detectors 5 and 7. The first frequency detector 5 detects the tracking control signal of the 16 KHz frequency component from the output signals from the mixer 4 and wave-shapes the detected tracking control signal. Then, the first level detector 6 detects the level of the resultant tracking control signal of the 16 KHz frequency component from the first frequency detector 5.
Also, the second frequency detector 7 detects the tracking control signal of the 46 KHz frequency component from the output signals from the mixer 4 and wave-shapes the detected tracking control signal. Then, the second level detector 8 detects the level of the resultant tracking control signal of the 46 KHz frequency component from the second frequency detector 7.
The output levels from the first and second level detectors 6 and 8 are applied to the comparator 9, which subtracts the output levels from the first and second level detectors 6 and 8 from each other. In accordance with the subtracted result, the comparator 9 outputs the tracking error signal to the microprocessor 10. In response to the tracking error signal from the comparator 9, the microprocessor 10 outputs the capstan motor control signal for correction of the mis-tracking of the video heads HD. As a result, the phase of the capstan motor M is controlled according to the control signal from the microprocessor 10.
In brief, in the case where the video heads trace the tracks of the magnetic tape, on which the high frequency signals are recorded, and where the video heads to the left or right of the corresponding tracks, the tracking control signals are generated according to the differences between the respective reference pilot signals from the reference pilot generator 3 and the amplified pilot signal from the amplifier 2. Then, the tracking error signal is generated based on the difference between the tracking control signals and, thus, corrects the mis-tracking of the video head.
The tracking error signal may have different values according to an installation error of the video head, a frequency characteristic of the amplifier, a property of the magnetic tape or a recording manner of a video signal. In this connection, the conventional automatic tracking control apparatus has a disadvantage in that an amplification gain of the tracking error signal is constantly adjusted by the microprocessor in spite of the different values of the tracking error signal, resulting in an inaccurate control of the phase of the capstan motor. The inaccurate control of the capstan motor phase results in a jitter and a flutter which degrade both picture reproduction and sound reproduction.